drm/amd/display: Fail atomic_check early on normalize_zpos error

[Why]

drm_atomic_normalize_zpos() can return an error code when there's
modeset lock contention. This was being ignored.

[How]

Bail out of atomic check if normalize_zpos() returns an error.

Fixes: b2615099

 ("drm/amd/display: Fix double cursor on non-video RGB MPO")
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org